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> So I'd suggest forgetting about this part.
If someone really wants this feature (relative seek of more than 2^63), he/she should open a new issue.
This issue remembers me a mktime() issue: mktime() may return -1 even if it is not an error. time_mktime() uses now a sentinel to detect an error:
 buf.tm_wday = -1; /* sentinel; original value ignored */
 tt = mktime(&buf);
 /* Return value of -1 does not necessarily mean an error, but tm_wday
 * cannot remain set to -1 if mktime succeeded. */
 if (tt == (time_t)(-1) && buf.tm_wday == -1) /* OverflowError */
For lseek, we can rely on errno. Try something like that:
errno = 0;
offset = lseek(...);
if (offset == (off_t)-1 && errno) /* error */
We can write a test using a sparse file... Or maybe a mmap object?